The present invention relates to retractable seals for rotary machines such as steam turbines, gas turbines, aircraft engines, compressors and the like.
Rotary machines such as steam and gas turbines used for power generation and mechanical drive applications, aircraft engines used for propulsion, and compressors used for pressurization are generally very large machines consisting of multiple turbine and compressor stages. In such machines, pressurized fluid flowing through the turbine and/or compressor stages passes through a series of stationary and rotary components. In a typical steam turbine, the stationary components may include a machine casing and a packing head, and the rotary component may be a rotor supporting a plurality of bladed wheels.
Annular seals mounted on the stationary components are used to control leakage of fluid along the path between the stationary and rotary components. In fact, the efficiency of the turbine is directly dependent on the ability of these seals to prevent such leakage. These seals can be radial or axial in orientation, and can be one of several kinds such as labyrinth packing seals, leaf seals, abradable seals, compliant plate seals, etc. Radial seals are often segmented (i.e., divided into plural arcuate segments which together surround the rotor) for assembly reasons and/or for displacement in the radial direction. While such radial, segmented labyrinth seals have proved to be quite reliable in steam turbines, their performance degrades over time as a result of transient events in which the stationary and rotary components interfere, rubbing the labyrinth teeth into “mushroom” profiles and opening the seal clearances. Similarly, near-contact seals such as brush seals and compliant plate seals can possibly contact the rotor during transient events leading to wear and heat generation. Heat generation, in turn, can cause rotordynamic instabilities, which are detrimental to the machine's operation and performance.
One means of reducing the negative effects of rubs or contact during transient events has been to employ a variable clearance “positive-pressure” (VCPP) arrangement, in which springs are used to hold the seal segments open at a large running clearance under no or low-flow transient conditions, when such rubbing is most likely to occur. During steady-state conditions, when the machine is typically operating at a higher load with higher fluid pressures, the ambient pressure around the seal segment overcomes the spring force acting to close the rings to a close running clearance. Examples of known variable clearance positive-pressure (VCPP) labyrinth seals may be found in U.S. Pat. Nos. 6,695,316; 6,022,027; 5,810,365; 5,603,510; 5,002,288; and 4,443,311.
Variable clearance positive-pressure arrangements, however, employ segmented seals that respond solely to the machine load. Once the machine reaches a design load and pressure, the packing ring segments close and remain closed until the machine load, and therefore the fluid pressure inside the machine, drops adequately. Thermal transients may persist, however, even after the design load has been reached. Therefore, it is ideally desired that the seal segments remain open until the thermal transients subside. Furthermore, VCPP seals are susceptible to rubbing in case of rotor vibrations during steady-state operation, when the seal segments are forced closed by the ambient fluid pressure. In such circumstances, current VCCP arrangements are not effective in avoiding rubs since they are passive systems.
A further enhancement to this concept is achieved via the “Active Retractable Seals” technology, which allows packing-ring segments to be retracted actively during an operating condition of the turbo-machine and not just start-up and shut-down. This is accomplished by providing a flow bypass that neutralizes the pressure drop across the seal segments in question. By eliminating or reducing the pressure drop across the seal segments in this fashion, the fluid-pressure force (which is radially inwards for a radial seal configuration) on the seal segments is effectively reduced and the seal segments are retracted (radially outward for a radial seal configuration) by preloaded springs. Exemplary prior art is described in U.S. Pat. Nos. 6,786,487; 6,655,696; 6,572,115 and 6,502,823.
There remains a need for reliable spring designs for both passive and active retractable seals that keep the packing ring segments open or retracted in the absence of a pressure drop across the packing ring.